1. Field of the Invention
This invention relates to data transmission in a wireless environment, and is specifically directed to a method for reliably passing modem handshaking information over a cellular radio link.
2. Summary of the Prior Art
With the introduction of cellular modem technology in the mid-1980's, personal computing has reached a new level of mobility. No longer is the traveling business limited to PSTN (public switched telephone network) outlets or specialized private mobile links in order to access data remotely. Instead, the cellular modems permit wireless data exchange between field personnel and a home office over conventional cellular radio networks which increasingly span the globe.
Data on cellular is especially attractive because of the immense international cellular coverage area, the high penetration of mobile cellular handsets into the marketplace, the relatively modest RF power amplifier requirements making it well-suited for a mobile environment, and the capability to relay both voice and modulated data reasonably well in full duplex mode. Also, startup and subscription costs are much lower than current wireless alternatives which require specialized data-only mobile transceivers having limited geographical access.
In operation, the cellular modem, which is typically coupled between the laptop or mobile computer and the mobile handset, communicates with a land based PSTN modem or another cellular modem through traffic channels granted to the mobile handset for the duration of the call. Ideally, the mobile station should appear as just another PSTN modem to the remote land modem. However, unlike conventional fixed network telecommunications, cellular radio is an inherently "dirty" data carrier, with perceived Bit Error Rates (BERs) approaching 2% (i.e., one error per 50 bits) at conservative 2400 baud transmission rates (by contrast, PSTN modems are unlikely to experience BERs exceeding 1.times.10.sup.-5 at this speed). This is because, as a low power line-of-sight radio technology, cellular signals are susceptible to fading, shadowing and even dropout because of changing environmental conditions which momentarily block the transmission pathways. Noise and multipath interference also can severely degrade the signal. These events can result in signal loss anywhere from one millisecond to greater than five seconds, at which point the cellular system clears the call. Signal disruptions for several hundred of milliseconds can also occur due to the signalling strategy of cellular radio, such as power stepping the mobile handset at approaches or moves away from a cell site and handoff when moving from one cell boundary to another.
Though transmission impairment experienced during any point of a data call can be catastrophic, errors encountered during the initial handshaking phase have the most significant impact, for it is at this point where signalling protocol error correction and data compression standards are established for the call duration. This is compounded by the fact that much of the initial modem handshaking runs "bare", (i.e., no error correction or compression is performed to insure compatibility with older land modems lacking such features). Plus, standard cellular-specific error protection techniques such as ARQ (automatic retransmission request for erroneous data) and FEC (forward error correction) introduce significant lapses in the data stream which could, at this initial stage, be misinterpreted as a carrier loss event. For example, cellular signal fade experienced in the initial connect tone exchange may cause a land modem to mischaracterize the cellular modem's handshaking statistics, resulting in a 1200 baud connection with no error correction or compression even though both modems may possess full V.42bis and MNP5/LAPM capabilities. A less than optimal data bottleneck is thus needlessly formed, if indeed a connection is established at all.
Prior art systems have addressed this problem by the addition of specialized hardware into the cellular network or through the use of specially adapted modems at the mobile stations and fixed-ends. For example, both the proprietary AT&T/Paradyne and the MNP10 protocols provide for reliable handshaking services. However, both the cellular modem and the fixed-end modem must recognize and support these protocols; otherwise, the handshaking sequence is left unprotected. Other systems, such as Vodaphone Ltd.'s VMACS (Vodaphone Mobile Access Conversion Service) cellular data gateway used in Great Britain's analog Total Access Communications (TACS) cellular system eliminates the need for over-the-air modem coordination and handshaking since all cellular data transmissions follow a predefined transfer rate and error protection (i.e. through well-known cyclic CLDC algorithms). Handshaking with the PSTN modem is performed exclusively by pool modems on the fixed-end side of the VMACS gateway. However, the VMACS system is disadvantageous because it requires additional system hardware (specialized CLDC modems at the mobile end base stations) and system signalling services not provided in the typical AMPS analog cellular system while limiting the mobile subscriber to only one type of cellular modem. Plus, the meager predefined data rates only permit forward transmission at 2400 baud and reverse transmission at 150 baud. With VMACS, therefore, mass data transmission simultaneously in both directions is not possible and hardware options are limited.
It is therefore desirable that a more flexible solution be found to optimize potential data throughput and reliability in a cellular environment. A variety of cellular modems and transmission protocols ranging from basic Bell 103 to V.42bis and beyond should be supported. To reduce cost and ease the equipment upgrade process, the desired solution should rely on additional firmware rather than hardware modifications whenever possible. It would also be desirable that the reliable handshaking functions be carried out in a manner transparent to the land modem yet remain compatible with non-upgraded cellular equipment. Finally, the desired techniques should be generally adaptable to a wide variety of analog cellular systems in current and projected use, including AMPS (Advanced Mobile Phone Service), TACS, JTAC (Japanese TACs), NAMPS (Narrowband AMPS) and NMT (Nordic Mobile Telephone).